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igcse physics notesprinted 123 · PDF 123
124 pagesrevision 46936
printed 123 · PDF 123
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radius of the orbit in metres (m) T= orbital period in seconds (s) This orbital period (or
time period) is dened as: 2πr T The time taken for an object to complete one orbit The
orbital radius r is always taken from the centre of the object being orbited to the object
orbiting
The Solar System The Solar System consists of: . The Sun . Eight planets . Natural and
arti cial satellites . Dwarf planets . Asteroids and comets The Sun & the Planets The
Sun lies at the centre of the Solar System The Sun is a star that makes up over 99% of
the mass of the solar system There are eight planets and an unknown number of dwarf
planets which orbit the Sun The gravitational eld around planets is strong enough to
have pulled in all nearby objects with the exception of natural satellites The gravitational
eld around a dwarf planet is not strong enough to have pulled in nearby objects The 8
planets in our Solar System in ascending order of the distance from the Sun are:
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Satellites There are two
types of satellite: Natural Arti cial Some planets have moons which orbit them Moons
are an example of natural satellites Arti cial satellites are man-made and can orbit any
object in space The International Space Station (ISS) orbits the Earth and is an example
of an arti cial satellite
Asteroids & Comets
Asteroids and comets also orbit the sun An asteroid is a small rocky object which orbits
the Sun The asteroid belt lies between Mars and Jupiter Comets are made of dust and
ice and orbit the Sun in a di erent orbit to those of planets The ice melts when the comet
approaches the Sun and forms the comet’s tail
Measuring Galactic Speed & Distance EXTENDED
Using Redshift Observations to Measure the Universe The change in wavelength of the
galaxy’s starlight due to redshift can be used to nd the velocity, v, with which a galaxy
(or any distant object) is moving away from Earth Using an equation to compare the
ratio of the expected wavelength with the observed wavelength, the velocity can be
found; differencebetweenactualwavelengthandexpectedwavelength actualwavelength =
speedofthegalaxy speedoflight This equation will not be directly examined but the idea
that the velocity of distant objects can be found from the redshift seen in easily
observed wavelengths is an important one Measuring Distance Using Supernovae
Redshift and CMB radiation allow various measurements of the Universe to be
accurately made Measuring distance is done using di erent methods A key method is
the use of standard candles, including supernovae Supernovaeare exploding stars